Refractive index measurement of absorbing and turbid fluids by reflection near the critical angle

Abstract

A method is described for measuring the specular optical reflectance R($) of the interface between a fluid and a glass prism, the incident and reflected light being in the glass, and the angle of incidence 4 being varied about the critical angle. Transparent, absorbing and some turbid fluids give R y ) close to theoretical predictions, and experimental reflectance data are analysed to give the real (n') and imaginary (n") refractive indices of the fluid. Other turbid fluids gave R(4) data that differed strongly from theoretical expectation. This is attributed to heterogeneity of the fluid on the optical wavelength scale, and problems of refractometly for such fluids are discussed.

Accuratein situmeasurement of complex refractive index and particle size in intralipid emulsions

“…It is noteworthy that, despite all the attention, a precise in situ determination of several important optical properties of intralipid emulsions, such as the particle size, refractive index, and attenuation coefficient (which is commonly expressed as the imaginary part of a complex refractive index [13][14][15][16][17][18][19][20][21] ), has continued to elude researchers. 28 The chief difficulty arises from the fact that intralipid emulsions, being highly turbid, have a large attenuation coefficient.…”

“…23,28 From the point of view of in situ complex refractive index measurement of highly attenuating tissue, it is attractive to consider reflectance-based methods since very little light transmits through. However, the most widely used reflectance-based method, which equates the point of maximum slope of the reflectance-versus-incident-angle curve with an effective critical angle, 3,[13][14][15]21 has been shown to be significantly inaccurate even after error-correction is attempted. 29 Other attempts to extract the complex refractive index by modeling the reflectance data either introduce extraneous fitting parameters (beyond the two parameters of interest, namely, the real and imaginary parts of the refractive index) resulting in overfitting of the data 19,20 or focus on only the critical angle region, which is just a small subset of the reflectance data.…”

“…16,17, and 18 where a small subset of the reflectance curve was focused on and previous works 5,14,15,20 where just ∼20 data points were fitted, here we fit the entire reflectance curve comprising ∼1000 data points. In further contrast to previous works 5, [19][20][21] we employ no other fitting parameters besides the two variables we seek to determine-the real and imaginary parts of the refractive index-thus avoiding ambiguity in the extracted values owing to overfitting. To the best of our knowledge, this is the first case where the entire reflectance-versus-incident-angle data-curve, spanning both TIR and non-TIR regimes, has been accurately described by a theoretical model for any highly turbid medium.…”

Refractive index dispersion measurement of absorbing layers from transmittance spectra

“…Photometric techniques [1][2][3][4] used to measure the optical constants of materials are easily adapted to obtain spectra of the optical constants and are suitable to be used with thin films. The accuracy is typically on the second or third significant digit.…”

“…If we suppose Ӷ n, and expand Eq. ͑2͒ in a power series of and neglect terms of order 2 we have Х / cos m where cos m = ͑1 − sin 2 i / n 2 ͒ 1/2 . Thus, m is given by the usual Snell's law with the real part of the refractive index of the slab.…”